The background of this invention is the domain of dimensional measurement, inspection, and position control. More specifically, the determination of the shape of a rotating body as well as the location of its spin axis is measured.
One application may described as follows: In magnetic recording media, for example as used in hard disk storage devices, information is written to and read from magnetic elements that represent digital bits. In order to increase the amount of information that can be stored within a given area, the size and distance between these magnetic elements may be reduced so that they may be more densely positioned. At the same time, in order to increase production volume and decrease production cost, the speed at which disks are written to and read from when preparing the disks for use by an end-user may be increased. For example, a disk format operation may be performed on one or more disks during the manufacturing process in preparation for use by an end-user. In any case, accurate information of location as a function of time of the spin axis of the disks is useful.
One way to increase disk production volume and decrease production cost is by increasing the speed at which the disks rotate so that more magnetic elements may be accessed within a certain amount of time, thereby yielding more completed disks within the same amount of time. Another way to increase disk production volume and decrease production costs is by performing the same operations on more disks simultaneously, thereby requiring less manufacturing equipment.
However, both of these methods require more energy, which in turn increases the noise and vibration in the disks' environment. The interferences caused by rapid disk rotation and other interferences may cause random radial displacement or eccentricity of the rotating disk, resulting in non-repetitive run-out. As a result, in combination with the increasingly small size and proximate positions of the magnetic elements, the non-repetitive run-out may interfere with the accurate writing and reading of information stored on the disks, during their various manufacturing phases.